The invention is a tuning pin for direct mounting on the string plates of mechano-acoustic pianos. Present-day pianos employ the same means for tensioning the strings that was used in the oldest known piano, an instrument built by Bartolomeo Cristofori in 1720; Metal pins, each a few centimeters in length and less than one centimeter in diameter, are driven into a slab of wood called a wrest plank or pin block. Each piano string is coiled around a pin in such a way that the tension of the string can be adjusted by rotating the pin about its central axis with a suitable tool. Advantages of this apparently anachronistic method are its simplicity and the low cost of the pins themselves, characteristics that may account for continued use in spite of many disadvantages, mostly relating to the use of a wooden pin block. The present invention seeks to eliminate the disadvantages by eliminating the pin block and by introducing certain novel and useful improvements. The advantages of the invention can best be understood if the problems associated with the use of conventional tuning pins in high quality grand pianos are first outlined.
Convention tuning pins for pianos are about 2.5" (6.35 cm), long are made of steel, and are generally cylindrical in form, but have flats about 0.63" (1.6 cm) long of tapering width on four sides near the upper end to permit engagement with a standard tuning lever, which has a socket tapered to fit the flats. A transverse hold about 0.06" (1.5 mm) in diameter is located at the lower end of the flattened portion to permit insertion of the end of a piano string. The lower part of the pin is threaded for about 1.5" (3.8 cm) of its length with shallow external screw threads of a type not common to any other product. Conventional tuning pins are available in the U.S. in the following standard diameters: 0.282" (7.16 mm), 0.286" (7.26 mm), 0.291" (7.39 mm), 0.296" (7.52 mm), 0.301" (7.64 mm), 0.306" (7.77 mm). Pins of 0.282" (716 mm) diameter normally are used in new pianos. The larger sizes listed are used for periodic replacement as smaller pins become loose.
High quality pin blocks are normally about 13/8" (3.5 cm) thick and usually are made of a number of relatively thin laminations of selected hardwood that are glued together face to face with a high-quality adhesive. The material and processing of the pin block are crucial to the tunability and longevity of a piano. U.S. Pat. Nos. 2,736,224, 3,091,149, 3,721,147, and 3,805,666 summarize the present art of manufacture of wooden pin blocks.
Pin blocks shaped approximately for the piano models in which they will be used are cut from large slabs of laminate. Each pin block is then hand fitted to a particular metal string plate by filing off small amounts of wood until the contour of the block is complementary to the contour of a mating flange on the string plate. This is done to prevent relative motion between the block and the plate, which could cause unstable pitch in the finished instrument.
After a pin block has been assembled to its plate, the tuning pin holes are located by center-punching through holes that are already in the plate. A precisely sized drill bit must be used, and close control of both rotational and axial bit speed is needed in order to produce cleanly-drilled holes of exactly optimum diameter. U.S. Pat. No. 2,736,224 teaches the importance of correct procedure. Errors in drilling can cause the pins to be too loose or too tight, the life of the pin block to be shortened, and tunability to be poor. Installing the pins is also a critical operation. If a pin is started with its axis at an angle to the axis of the hole, the hole can become enlarged, causing a loose pin. Abnormally loose or tight pins also can be caused by incorrect pin diameter. The normal diametral tolerance for tuning pins is .+-.0.001" (.+-.0.025 mm), but pins are sometimes out-of-tolerance or out-of-round. Pins of irregular size produce non-uniform torque.
Dirt, oil, or other foreign matter may enter the pin holes along with the pins, and can cause irregular torque or "stick-slip", a ratchet-like motion of the pin that occurs if the torque required to initiate pin motion is significantly higher than that needed to sustain the motion. Irregular torque or excessive stick-slip can prevent accurate tuning. Irregularities in the wood itself can cause uneven torque. Wide variations of torque from pin to pin frustrate good tuning results. Wooden pin blocks wear out. Because of wear due to tuning and/or deterioration due to an unfavorable storage environment, pin torques gradually decline and eventually may become so low that the piano either cannot be tuned or will not stay in tune. The useful life of a piano is often limited by the life of its pin block.
Piano tuners generally prefer pin torque to be in the range between 75-125 inch-lb (8.48-14.1 Newton-meters). However, if a pin block is drilled to give torque values this low in a new piano, the pins may become too loose to hold tuning long before the normal life span of the instrument has been reached. Therefore, manufacturers normally make the initial torque of the pins higher than is optimum for tuning. Torque cannot be adjusted once the pins have been installed. There are no acceptable effective ways to increase pin torque except by removing the strings and pins and installing oversize tuning pins, or by replacing the entire pin block. Such operations are costly and disruptive to the stability of the instrument. In order to prevent loose pins, some manufacturers make the pins extremely tight when the piano is new. This practice can produce pin torques of 200-300 inch-lb (22.6-33.9 N-m), and pianos that remain difficult to tune accurately for many years after they leave the factory.
That piano technologists have long been aware of the disadvantages outlined above is indicated by the number of patents relating to tuning means for pianos that have been directed toward eliminating the pin block. The following may be cited: U.S. Pat. Nos. 375,150, 388,720, 420,914, 488,843, 1,094,653, 1,615,228, 3,156,151, 3,757,026; German Pat. Nos. 227,526, 340,968, 381,384, 444,828; British Pat. Nos. 21,136, 301,607; French Pat. No. 2,479,523. Despite all previous attempts to displace it, the simple tuning pin and wooden pin block combination still is the only tensioning means for piano strings that is in production use at the present date, so far as is known to the applicant. Further, the only means known to applicant ever to have achieved significant production use in pianos was disclosed by U.S. Pat. No. 388,720, issued to J. P. Richardson on Aug. 28, 1988. This means was used for a time by one piano company but was abandoned, perhaps because accurate tuning was more difficult, excessive space in the piano was required, stringing was difficult, or cost excessive.